Electron emitting system



- July 28, 1959 M. VON ARDENNE 2,397,396

7 ELECTRON EMITTING SYSTEM' '7 Filed Jan 2 4, 1956 v I 2 Sheets-Sheet 1 sl/ncuavsn gnu/540,08 516.3

July 28, 1.959 M. VON ARDENNE I Q 2,397,396

ELECTRON EMITTING SYSTEMP I FiledJan. 24, 1956 I 2 Sheets-She'et 2 m we g: 4 24 2 INVENTOR. HlnfN-Il Mm Hrn/nm;

atcnt fiti 2,897,396 Patented July 28, 1959 ELECTRON EMITTING SYSTEM Application January 24, 1956, Serial No. 561,064

Claims priority, application Germany May 10, 1955 10 Claims. (Cl. 315--3) The present invention relates to an electron emitting system, and more particularly to an electron emitting system having a cathode and an anode.

The present invention provides an electron gun for producing a very high density electron beam. It will be understood by those skilled in the art that such high density electron beams are useful in many types of electron beam or cathode ray tubes.

It is an object of the present invention to obtain a high density of the electron current.

It is another object of the present invention to obtain a simple stabilization of the electron current.

It is a further object of the present invention to obtain a directed electron beam the intensity of which surpasses by several orders of magnitude the intensities hitherto obtained in the art.

Other objects and advantages of the present invention will become apparent from the following detailed description thereof in connection with the accompanying drawings showing, by way of example, some embodiments of the present invention.

In the drawings:

Fig. l is a sectional view on an enlarged scale of a first embodiment of the present invention,

Fig. 2 is a sectional view of the same embodiment of the present invention after a certain period of time of the operation thereof,

Fig. 3 is a sectional view on an enlarged scale of a modified embodiment of the present invention,

Fig. 4 is a sectional view on an enlarged scale of part of another embodiment of the present invention,

Fig. 5 shows in section part of an arrangement for manufacturing the embodiment shown in Fig. 4,

Fig. 6 is a diagrammatic sectional view of a further embodiment of the invention,

Fig. 7 shows on an enlarged scale the right hand part of Fig. 6,

Fig. 8 shows diagrammatically a still further embodiment of the invention, and

Fig. 9 is a diagram of still another embodiment of the present invention.

Referring now to the drawings and first to Fig. 1, an evacuated vessel 10 contains a residual gas having a pressure of 10- to 10" Torr. In the vessel 10 a main cathode 12 having preferably the shape of a rod having a diameter of about 3 1 is arranged so as to be surrounded by an auxiliary cathode 14. The main cathode is provided with a main electron emitting surface 16 formed by a material having a relatively low work function whereas the auxiliary cathode 14 has an auxiliary electron. emitting surface 18, which is formed by a material having a relatively high work function. For instance the main cathode 12 may consist of tantalum or magnesium whereas the auxiliary cathode 14 and the electron emitting surface 18 thereof may consist of platinum having a work function value of approximately 5 volts. At a 'impinges on the anode.

switching means (not shown).

distance d of, for instance, 10- centimeters shown on an exaggerated scale, an anode 20 is arranged opposite the main and auxiliary cathodes 12 and 14.

The operation of this device is as follows:

If a voltage amounting to for instance 5.10 volts is maintained between the anode 20 and the main cathode 12 5 and the auxiliary cathode 14, a beam 22 of electrons will be emitted by the electron emitting surface 16 of the main cathode 12 having a relatively low work function,

whereas the emission of electrons by the auxiliary cathode 14 will be negligible. The beam 22 of the electrons After a certain time the impinging electrons 22 drill a small hole 24 (Fig. 2) in the anode 20 and pass the same as an electron beam 26. The stabilization of the space current is effected by external The main electrode 12 is preferably formed as a very fine wire surrounded by an envelope or auxiliary cathode 14 consisting of platinum and provided with heating means (not shown). The manufacture of the cathodes 12, 14 may be effected, if desired, by the Wollaston wire method known in the art. The embodiment shown in Figs. 1 and 2 has the disadvantage that the total field current passing through the auxiliary electron emitting surface 18 is only a little smaller than the current of the main electron emitting surface, 16 and that for stabilizing the electron current externalswitching means (not shown) have to be provided.

. Referring now to the embodiment shown in Fig. 3 of the drawings in which the gas envelope has been omitted the main cathode 112 is formed as a wire having a diameter of 1,11. and the electron emitting surface thereofis formed by a calotte 116 so that the beam 122, 126 emitted by the main electron emitting surface or calotte 116 and passing the hole in the anode is larger by many orders of magnitude than the emission of the auxiliary electron emitting surface 118 of the auxiliary cathode 114. The main cathode 112 and the main electron emitting surface 116 the electric field strength acting on the same is considerably increased. The stabilization is effected in a very simple manner by the field screening effect of the space charge of the electron beam 122. However, this eifect is only reached at densities. of the emission current of the order of magnitude 10 a./cm.

so that in order to maintain the total current of emission at a relatively small value the calotte 116 should have a diameter equaling about In. The manufacture of a well defined calotte of about I diameter and consisting for instance of tantalum on the polished and highly cleaned platinum surface 118 is not a simple matter. However this calotte may be manufactured according to a further embodiment of the present invention shown in Figs. 4 and 5 in the following manner:

Referring now to Fig. 4 of the drawings, 214 is a platinum electrode carrying a calotte 216 having a diameter amounting for instance to In. In order to manufacture suclra calotte the platinum cathode 214 is arranged in an evacuated vessel (not shown) and a foil 220 (Fig. 5) is arranged at a very small distance from the same. The foil 221) has a small boring 222 of a diameter amounting for instance to 3, and is arranged in front of the platinum electrode 214 the frontal surface of which is highly cleaned. A vapor jet indicated by the arrows 2'26 and generated by vaporizing the material of the main cathode within a furnace or a crucible (not shown) being enclosed in the same vacuum as the platinum electrode 214 and the foil 220, is directed against the latter so that microscopical particles 228 of tantalum are deposited on the side of the foil 220 turned away from the platinum electrode 214. Part of .the tantalum passes through the boring 222 of the foil .220 and is deposited as a calotte "216 on the platinum electrode 214.

Referring now to Figs.'6 and 7 of the drawings, an

arrangement is shown in which the boring in the foil is with a central boring 310 through which the electrons indicated by the reference numeral 312 enter an elongated cylinder 314 forming preferably one piece with the anode 308 so as to form a crossover 316 having preferably a diameter of 0.1 millimeter. The cylinder 314 is grounded at the farther end thereof and ends in a magnetic reducing lens generally denoted by the reference numeral 318 which will be described presently more in detail. The cathode 300, the Wehnelt cylinder 302 and the anode 308 form a three-electrode system for generating the beam of electrons 312 and shall not be described further in detail since it is Well known in the art such as the electron microscopy. However, it should be understood that the cathode 300 may be formed, if

desired, as a cathodic system including a main cathode and an auxiliary cathode, as shown in Figs. 1-3, the main cathode having a relatively low work function and consisting preferably of tantalum or magnesium, the auxiliary cathode having a relatively high work function and consisting preferably of platinum.

The magnetic reducing lens 318 arranged at the end of the cylinder 314 opposite to theanode 308 includes a hollow pole shoe 320 consisting of magnetic material which encloses an electric winding 322. The inner annular part 324 of the hollow pole shoe 320' is in contact with the cylindrical part 326 of a second pole shoe having a conical part 328 forming an extension of the cylindrical part 326. Between the pole shoes 320 and 328 a diaphragm 330 is arranged having a central boring being in alignment with the central borings 322 and 334 of the first and second pole shoes 320 and 326, 328. Opposite to the boring 332 of the first pole shoe 320 and on the side thereof turning away from the diaphragm 330 a foil 336 is arranged. The foil 336 is to be provided with a fine boring 338 being in alignment with the bor ings 332 and 334, by the central part of the electron beam 312 passing through the borings 332 and 334. The fine boring 338 coincides with the focal point of the electron beam 312 where the intensity measured in watts per square centimeter is high. The dimensions are chosen so that an image of the crossover 316 of the electron beam 312 is formed on the foil 336, the image being a circle having a diameter of a few microns at a temperature of about 10 C., so that a very fine boring 338 is made in the foil 336 by the focussed electron "beam 312.

The very small distance between the cathode and the 'anode of the rnicrosystem may be controlled in a parti'cularly convenient manner by changing the temperature and thus the thermal expansion of the parts of the microsystem. For instance the temperature of the main cathode 12 shown in Figs. 1 and 2 or 112 shown in Fig.3 may be changed by changing that of the platinum aux- "iliary cathode between 500 and 1000 C. and thus the distance d separating the surface 16 of the cathode from the anode 20 may be finely adjusted so as to adjust the electric field strength E produced by the voltage V.

Fig. 8 of the drawings shows another way of accomplishing this result. The voltage U existing between the anode 408 and the main and auxiliary cathodes 412 and 414 may be adjusted by means of a variable resistor 400 inserted in series with the voltage U The beam 422 of electrons impinges on a screen 424 maintained by the connection 426 at the same positive potential as the anode 408. p

In the embodiment shown in Fig. 9 of the drawings the resistor 400 connected in series to the voltage U shown in Fig. 8 is replaced by a circuit comprising a voltage source U having a negative terminal 430 'connected with the auxiliary platinum cathode 414. The positive terminal 432 of the voltage source U is connected to the plate 4340f a tube 436 having a grid 438 connected to the negative output terminal of a DC. amplifier 440 the positive output terminal of which is connected to the cathode 442 of the tube 436. The input terminals 444 of the DC. amplifier 440 are fed by ='a resistor 446 connected between ground and one terminal of a voltage source 448 of, for instance, 5000 volts, having the other terminal thereof connected to the plate 434 through a resistor 450.

The following tables contain numerical data for preferred embodiments of my invention for electron guns or the type illustrated in Fig. 3.

Microsystem I Stabilization By External Means Mzcrosystem Il Main Auxiliary Cathode Cathode Ta (Mg) PtI (PtII) Suction Distance '11 cm 10- Z10- Anode Voltage U kilovolts .25 to 30 25 to 30 Suction Field Strength E10 volts per cm-.- 7 to 8. 5 2. 5 Efieotive'Cath'ode Surface 'F squa're cm" 10 3.10- Emission Current Density in;

amps. per square cm 10 to 10" 10- Emission Current in rnil1iamps 10 to 3.10-

By Space Charge Stabilization Hitherto the high electron current densities of the field emission (j l-0 to 10 amps. per square cm.) could be realized in a stationary manner only when points consisting of fine highly polished tungsten with radii of curvature amounting to 0.02 to 1,11. so as to render the emitting-surface very small were used in a very good because the electrons are emitted from the point into a very large solid angle are where a is the angle of aperture. The emission from completely or approximately vplane'cathodes into a small solid angle 'n'ot has not been accomplished hitherto, not even at an impulse-like load The development of a field beam value failed. owing to the probability of the formation of subsidiary centers of emission due to impurities 3. .4. il? 1 which increases with the surface, and due to the excessively steep course of the characteristic of the field emission.

Fluctuations of the field emission current by many orders of magnitude are due to small changes of the local work function by changes of the surfaces layer, diffusion phenomena along the surface, condensation products from organic vapors, impacts of ions in dependence on the'emitted current, and so on, and particularly to changes of the local surface geometry.

The invention overcomes these difficulties and comprises essentially the application of an electrostatic suction field with preferably parallel course of the field lines.

According to the invention secondary centers of emission are largely avoided by means of a metal envelope having a small surface which surrounds the center of field emission and is appreciably cleansed from insulating films or particles. This envelope is preferably manufactured from a metal having an as large as possible work function. This condition may be combined, for instance, with the requirement of keeping clean from insulating films or particles, when the envelope is manufactured from platinum, and the platinum is cleaned by heating to, for instance, 600 to 1000 C. either continually or in suitable periods of time so that the platinum oxide is decomposed and evaporated and the insulating organic condensation products are transformed into conductive films or carbon.

The field emission current is stabilized, according to the invention, by means of a control mechanism which reduces the field strength E acting on the field emission cathode as soon as the field current density exceeds or falls below a predetermined value.

Considering the time of transit of the ions formed by the field current, this control mechanism has preferably a time constant smaller than 10' seconds. For example, the control may be effected in at a low capacity construction of the electrodes, by a high ohmic series resistor which reduces the voltage between the participating electrodes as soon as the field current increases strongly. If necessary, the time constant of the control mechanism may be always maintained suificiently small by means of variable-gain amplifiers which, however, involve a certain expenditure. Considerably more economical is a stabilization of the field electrode current by natural space charge. A calculation has shown that an automatic stabilization by the field screening effect of the space charge is efitected at accelerating voltages amounting approximately to 5.10 volts at suction distances d-10- cm. for emission current densities of approximately j =l amps. per square cm. The probability of an occurrence of centers of secondary emission may be maintained sufficiently small, as was shown by experiments, by reducing the secondary surface to the minimum value required for reducing the divergence of the field lines, and by utilizing a platinum highly purified by a permanent heating.

Owing to the form of the suction field rendering the electron radiation parallel, the extremely high emission current densities, and the small dispersion of the velocity of field emission electrons, directional beam values are obtained which surpass by several orders of magnitude the directional beam values of the customary electron radiators.

I have described in detail hereinabove and shown n the drawings preferred embodiments of an electron emitting system. However, it should be understood that systems differing from the ones described heremabove and shown in the drawings, by structural changes, modifications, and substitutions of equivalents are to be regarded as falling within the protection afforded by the appended claims.

I claim:

1. A field emission electron emitting system, comprising a cold cathode having a surface having an area of the order of 10' square cm., a main cathode located at a central portion of said surface having an area of the'order of 10* square cm. having a relatively low work function and the remainder of said surface having a relatively high work function, a plane, anode juxtaposed to said surface of said cathode at a distance of the order of 10- cm. therefrom, the spacing between said surface and said anode being substantially constant -in the vicinity of the main cathode said anode having an .opening commensurate with and opposite to the portion of the cathode which has a low work function, and means for producing a parallel electrostatic suction field between the anode and the cathode of the order of 10 volts per cm. whereby the suction field extracts electrons from the main cathode portion having a current density of the order of 10 amperes per square cm. and directs the extracted electrons through the opening in the anode.

2. A field emission electron gun comprising a cold cathode and an anode, means including said cathode and anode and a source of voltage connected therebetween for producing a substantially parallel electric field between the cathode and the anode having a sufiicient strength to cause electron emission from the cathode, and means including said cathode and anode for reducing the strength of said electric field immediately adjacent the cathode in response to an emitted cathode current density greater than a predetermined value and for increasing the strength of said electric field when said current density falls below said predetermined value, whereby the cathode emission is stabilized, wherein said cathode has a main field emission cathode surface formed by a material having a relatively low work function and a secondary cathode surface adjacent and surrounding the main cathode surface and formed by a material having a relative high work function, the secondary cathode surface being shaped so as to focus the lines of electric force to form a substantially parallel electric field.

3. An electron gun according to claim 2 wherein the emission stabilizing means include a resistor connected in series with said source of voltage between said anode and cathode.

4. An electron gun according to claim 2 wherein said emission stabilizing means include a direct current amplifier connected between the anode and cathode in series with said source of voltage, said amplifier having a time constant smaller than 10- seconds.

5. An electron gun according to claim 2 wherein the spacing between the cathode and the anode and the voltage provided by said source have such values that the space charge produced between the cathode and anode has a sulficient electron density to effect the stabilization of the emission current.

6. An electron gun according to claim 2 wherein the size of the electron emitting surface of the cathode is such as to produce a current of the order of 0.1 to 10 milliamperes.

7. An electron gun according to claim 2, wherein the secondary cathode surface has an area only large enough to produce the required parallelism of the electric field, said secondary cathode surface being formed of polished platinum and said main cathode surface being formed of tantalum.

8. An electron gun according to claim 2, wherein the main cathode surface consists of a vapor deposited calotte of said low work function material.

9. An electron gun according to claim 2 wherein the distance between the anode and the cathode is a few microns and means for heating said cathode and anode electrodes to cause thermal expansion thereof in order to control the distance therebetween, thereby controlling the value of the emission current at a given anode voltage.

10. A field emission electron gun comprising a cold cathode and an anode, means including said cathode and anode andaa source of-voltage connected :therebetween for producing :-'a *substantially zparallel electric :field :between the cathode and the "anode ihaving "a 'silflicient strength to cause electron emissicn' from the cathode, and means including sa'id cathode and -anode aim- V reducing :"-the strength of said electric field immediately adjacent the cathode in response to an emitted cathode current density greater than a predetermined value and for increasing thestrength of said electric field when said current density-ialls-below :said predetermined value,

whe'reby the *cathode emission is stabilized, said anode being a metallic foil thinenough to be punctured by the '2,22:7;017 "Schlesinger 'Dec. '31, "1940 ,272,353 Rus'ka Feb. "10, 1942 2,363,359 "R'aiho NOV. 21, 1944 2,509;O53 :Calbik 'MBLY 23, 1950 2,540,950 Cock June '2, 1953 ,2,78'6;955 Trdla'n Mar. 26, 195-7 

